Hearing aids are portable hearing instruments serving to provide hearing assistance to the hearing-impaired. In order to meet the numerous individual requirements, different types of hearing aid are available, such as behind-the-ear (BTE) devices, hearing aids with external receiver (RIC: receiver in the canal) and in-the-ear (ITE) hearing aids, e.g. also concha or completely-in-canal (CIC) instruments. The hearing devices listed by way of example are worn on the outer ear or in the auditory canal. However, bone conduction hearing aids, implantable or vibrotactile hearing aids are also commercially available. In these cases, the damaged hearing is stimulated either mechanically or electrically.
The basic components of a hearing aid are essentially an input transducer, an amplifier and an output transducer. The input transducer is generally a sound pickup device, e.g. a microphone, and/or an electromagnetic pickup such as an induction coil. The output transducer is mainly implemented as an electroacoustic transducer, e.g. a miniature loudspeaker, or as an electromechanical transducer such as a bone conduction receiver. The amplifier is usually integrated in a signal processing unit. This basic design is shown in FIG. 1 using the example of a behind-the-ear hearing aid. Installed in a hearing aid housing 1 for wearing behind the ear are one or more microphones 2 for picking up sound from the environment. A signal processing unit 3 which is likewise integrated in the hearing aid housing 1 processes the microphone signals and amplifies them. The output signal of the signal processing unit 3 is transmitted to a loudspeaker or receiver 4 which outputs an audible signal. The sound is in some cases transmitted to the wearer's eardrum via a sound tube which is fixed in the auditory canal using an earmold. The hearing aid and in particular the signal processing unit 3 are powered by a battery 5 likewise integrated in the hearing aid housing 1.
Sound signals which are picked up by one or more microphones of a hearing aid are usually decomposed into sub-band signals for further processing. For this purpose, one or more frequency-selective digital analysis filter banks (AFBs) are normally used, thereby obtaining K>1 sub-band signals. After decomposition, sub-band-specific signal manipulations can be performed. In the case of hearing aids these are in particular amplifications in the individual sub-bands. The manipulated sub-band signals can subsequently be re-synthesized by means of a digital synthesis filter bank (SFB).
High-quality filter banks in hearing aids are subject to certain requirements. Thus, for example, in the lowest bands a channel bandwidth of approximately 250 Hz is required. Apart from that, the band spacing should more or less conform to the Bark scale. In addition, a channel number of at least 22 is desirable. Noise components caused by aliasing must be reliably below 60 dB. Because of the intensive sub-band processing (particularly the high amplification required for compensating for hearing impairment) in hearing aids, conventional methods for eliminating aliasing are ineffective. The filter banks must therefore be basically “non-critically” sampled. Moreover, the group delay (for both AFB and SFB) must be well below 5 ms and the group delay distortion must not exceed certain limits. Particularly for high frequencies, group delay must be kept as low as possible, which constitutes a significant limiting factor for the filter bank.
Publication DE 698 33 749 T2 discloses a filter bank arrangement and a method for filtering and separating an information signal in different frequency bands for audio signals in hearing aid devices. In an analysis filter bank, an input signal is decomposed into sub-band signals. The sub-band signals are amplified if necessary and combined into an output signal in a synthesis filter bank. The signals are oversampled in the analysis filter bank.